• Preheat oven to 350 degrees.

Crust

• Mix graham cracker crumbs and sugar in a bowl.
• Melt butter and pour into the graham cracker crumb mixture. Using your fingers or the back of a tablespoon, press into the bottom and up the sides of a 9-inch pie plate to ¼" thickness all around.
• Bake 6-8 minutes until very lightly browned. 

Pie Filling

• Separate egg yolks, reserving the whites for meringue.
• Mix well with sweetened condensed milk until batter has thickened.
• Add Key lime juice and mix until thoroughly combined.
• Pour pie filling into pie crust, and bake for 20 minutes.
• Cool at room temperature for 45 minutes, then chill in oven for 3 hours.
• Top with meringue. Place in 450 degree oven for a few minutes, just until tips of meringue have lightly browned. (You could also serve plain or with whipped cream topping, if preferred.)

Meringue Topping

• Preheat oven to 450 degrees
• Using an electric mixer, beat the egg whites with cream of tartar until soft peaks form (when tips of egg whites fall back on to peaks.)
• Mix sugar and water in a saucepan and heat to a boil on stovetop, stirring frequently. Measuring with a candy thermometer, bring mixture to a temperature of 240 degrees Fahrenheit. 
• Very slowly, add the sugar mixture to the egg whites, while mixing at medium speed, being careful not to splash yourself with the hot sugar. (A stand mixer with a pouring shield works well for this step.)
• Continue mixing at medium-high speed until stiff peaks form (peaks should stand up straight when you pull a spatula up through them.) Be patient; this can take up to 10 minutes. 
• Generously top the chilled Key lime pie with meringue. Dip the spatula up and down in the meringue to pull up decorative peaks.
• Place pie in oven and bake, watching constantly, just until top begins to brown. When the meringue starts to brown it browns quickly, so remove it promptly or the sugar will burn. This step takes only a couple of minutes.